LINK - BOKU TO KIMI NO AIDA

Link - Boku to Kimi no Aida summary:

After enrolling in a college, Mitsu begins living in a boarding house in hopes of living a peaceful life. But the people whom he lives with are made up of different personalities and they had a welcoming party for him. But the next day when he wakes up, he finds himself lying next to his next door neighbor... The novelist, Tsuburaya. Moreover, Mitsu can feel this throbbing pain coming from his butt, what could be the meaning of this...?